When facilitating calls (either private or group calls) in a communication system such as a two-way radio system, a “late entry” condition may occur. A late entry condition occurs when all or a portion of data embedded in a header signaling is lost during a transmission. Unfortunately, known procedures used during a late entry condition ultimately lead to degradation in performance by an end user.
For example, in accordance with European Telecommunications Standards Institute-Digital Mobile Radio (ETSI-DMR) standard (e.g., ETSI-Technical Specification (TS) 102 361 standard), a transmission includes at least one header burst that precedes a plurality of voice bursts, with the voice bursts comprising a voice superframe. The transmission also includes embedded link control (LC) data as part of the standard protocol. Since the embedded LC data indicates attributes about the transmission such as call type (group or individual call), transmitting unit's identity, identity of the destination such as talk group identifier (TGID), subscriber unit identity (SUID), privacy encryption status such as privacy on/off, etc., the embedded LC data could be used to facilitate late entry when a receiving device misses the at least one voice (and/or Privacy) header burst. However, no method currently exists that effectively uses this embedded LC data to improve performance during a late entry condition.
More particularly, where an intermediary device (such as a base radio repeater that repeats transmissions between a transmitting device and an end user device) is involved in a late entry condition, the intermediary device typically repeats the transmissions to the end user device without first recovering and Forward Error Correction (FEC) decoding the LC data, which leaves this function to be performed at the end user device. A shortcoming with this approach is that without error correction of the embedded LC data at the intermediary device, uplink errors (from the transmitting unit to the intermediary device) and downlink errors (from the intermediary device to the end user device) will net together making LC recovery at the end user device much more difficult, thereby, leading to degraded late entry performance at the end user device.
A further shortcoming with the approach is that the end user device typically must satisfy a number of conditions during a late entry condition before it can unmute and begin interpreting received data. The latency delay experienced while these conditions are satisfied may cause the device to miss portions of the voice transmission. Turning again to an ETSI-DMR transmission, the conditions that the end user device must satisfy to unmute are as follows. First, the end user device detects presence of a radio frequency (RF) carrier. Additionally, a synchronization (“sync”) presence is detected such as a data/control burst or a voice burst that includes a sync word. A timeslot is identified and a color code validated, which indicates that the end user device is on the correct system or site. Finally, the entire embedded LC data is extracted, FEC decoded and validated in order for the device to unmute to begin interpreting the received signal.
This extraction, FEC decoding and validation takes some time due to the format of the embedded LC data, which includes 72 bits of information along with a 5-bit checksum that is protected with a block product turbo code (BPTC) scheme, resulting in 128-bits of LC data. That 128-bit result is then interleaved and divided into four LC fragments which are embedded into four voice bursts of the voice superframe, for transmission. Thus, the voice data in a major portion of a transmitted voice superframe cannot be heard because the end user device is busy validating the LC data, and resources will be reserved on a downlink channel for at least an entire length of the voice superframe regardless of whether the transmission is a valid transmission for the end user device to join.
Therefore, a need exists for a method and device that addresses at least some of the shortcomings of past and present techniques used for call set-up when a voice late entry condition exists.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments. In addition, the description and drawings do not necessarily require the order illustrated. Apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood elements that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments.